1. Field of the Invention
The present invention relates to a bubble column reactor and a process to be carried out therewith for the preparation of ethylene glycol carbonate (EGC) or propylene glycol carbonate (PGC) by reaction of CO.sub.2 with ethylene oxide (EOX) or propylene oxide (POX) in EGC or PGC already present as reaction medium. The execution of the said process in the bubble column reactor provided according to the invention proceeds particularly mildly for the reaction partners and the catalyst to be used, since side reactions of the EOX or POX used can be substantially minimized. This results through a high selectivity of the reaction of EOX or POX with CO.sub.2. From this there results an energy- and material-saving process. The reaction is preferably carried out adiabatically.
2. Description of the Related Art
The preparation of EGC or PGC from CO.sub.2 and EOX or POX, respectively, has long been known. Many publications in this area relate especially to the use of certain catalysts. A detailed presentation of this can be found in Fette, Seifen, Anstrichmittel 73 (1971), 396. Additional publications are, for example, Ind. Eng. Chem. 50 (1958), 767, Chem. Ing. Techn. 43 (1971), 903 and German Offenlegungsschrift 28 55 232 (equivalent to U.S. Pat. No. 4,314,945).
The process described in Ind. Eng. Chem. (loc. cit.) for the preparation of EGC or PGC in the presence of tetraethylammonium bromide as catalyst is carried out at 180.degree. C. and 100 bar in a pumped circulation reactor. This pumped circulation reactor is composed of a tube, which serves as a reaction section and through which the reactants and the carbonate serving as reaction medium flow downwards, and of an external circulation, in which is situated a heat exchanger for cooling the circulated liquid to 50.degree. C. The feed of the EOX or POX and the catalyst, dissolved in the corresponding carbonate, is carried out at the top end of the reaction tube in which is located a thin layer with ceramic rings as a mixing zone. The further mixing of the reactants in the reactor is effected by the pumped circulation. CO.sub.1 is added at the bottom end of the reaction tube at a 6 to 7% excess, based on the alkylene oxide. The product stream from the pumped circulation reactor is conducted through a further tubular reactor which serves as a secondary reactor for completion of the conversion of the ethylene oxide and is likewise operated at 180.degree. C. and 100 bar. This tubular reactor described has the disadvantage that, in addition to the in any case drastic conditions, temperature peaks can occur in the reaction section which have a product-damaging action. These temperature peaks, without particular precautions, can exceed 200.degree. C. In order to counteract these temperature peaks, a high degree of back mixing is provided. The spatially adjacent metered addition of alkylene oxide and catalyst with simultaneously separate feed of the reaction panner CO.sub.2 leads to high local excesses of alkylene oxide. This type of addition and back mixing in the reaction section favor the side reactions of the alkylene oxide and, in combination with the temperature peaks described, represent further disadvantages of these reaction conditions. Because the reaction of CO.sub.2 with EOX or POX is highly exothermic, the reaction must be restricted in the first reactor and completed in a secondary reactor.
In the reactor described in Fette, Seifen (loc. cit.) and Chem. Ing. Techn. (loc. cit.), which is composed of a single tube without internals and without product circulation, CO.sub.2 and EOX or POX are reacted together at 80 bar and 190.degree. to 200.degree. C. in the presence of a catalyst dissolved in the carbonate to be formed and the heat of reaction is removed with the aid of a heat transport medium circulating in counter-current which is in turn cooled by water. Even with this type of reaction conditions, peak temperatures are obtained of up to 220.degree. C. in the reactor, which have a product-damaging effect, which is explicitly referred to in Fette, Seifen (loc. cit.); such temperature peaks are difficult to master, in particular in the case of an industrial plant. The entire heat of reaction is removed unutilized under these reaction conditions. Use would be achieved in a better manner through adiabatic reaction conditions, in which the entire heat of reaction liberated is absorbed by the reaction mixture itself. However, with exothermic reactions this always leads to an increase in the temperature of the reaction mixture, the harmful influence of which is referred to by the publications mentioned. Therefore, adiabatic reaction conditions are considered to be not technically realizable in Fette, Seifen (loc. cit.). As also in the process described further above, the conversion must be completed in a secondary reactor. Even in the reactor described without internals and without external product circulation, because of local alkylene oxide overconcentrations with respect to CO.sub.2, side reactions of the alkylene oxide can take place which lead to a decrease in the selectivity of the reaction and thus to a decrease in the yield.
In the said German Offenlegungsschrift '232, various combinations of flow tubes and pumped circulation reactors which are operated at 10 to 50 bar and 100.degree. to a maximum of 200.degree. C. are described as reactor for the preparation of alkylene carbonates from alkylene oxides and CO.sub.2. The carbonate to be formed serves in all reactor sections as reaction medium and represents there in each case 85 to 99.6% by weight of all substances present in the reactor. The starting materials CO.sub.2 and alkylene oxide are metered into the first reaction zone of such a combination together with the catalyst dissolved in the carbonate to be formed. CO.sub.2 is fed in this case only in a slight molar excess with respect to the catalyst. The components used for the reactor combinations used are stirred tanks, with external circulation for product cooling, and flow tubes. These reactor components, independently of their number and sequence within the combination, have in common that in them in each case only a partial conversion of the alkylene oxide can take place, since otherwise the heat problem of the highly exothermic reaction cannot be mastered. Only by such a complex connection of the reactors with coolers and execution of partial conversions can, in total, a conversion rate of alkylene oxide of about 99.5% and a carbonate selectivity of about 99% be achieved.
There was therefore the desire to have available a simple reactor, which can be enlarged to any scale, for the complete and selective reaction of alkylene oxides and CO.sub.2 to give the associated alkylene carbonates, which can dispense with secondary reactors, complex cooling systems or a complex coupling of the reactor components and coolers. In such a reactor, moreover, the formation of harmful temperature peaks should be able to be avoided. A supplementary desire consists in the highest possible utilization of the exothermic heat of the reaction, for example in the context of the work-up of the reaction product or to generate process steam.